This invention relates to cathode ray tubes (CRTs) for use in oscilloscopes, storage oscilloscopes, etc., and more specifically to a CRT having a novel electron lens system of two electrode configuration for amplifying the deflections of the electron beam, which dispenses with the familiar mesh that has been employed to obtain good display characteristics.
The postdeflection acceleration or postacceleration CRT has been known which employs a planar or domed mesh and a postaccelerating electrode on the inside surface of the bulb or envelope for creating an accelerating field designed to increase the velocity of the beam electrons after they have traversed the deflection fields. Thus postaccelerated, the beam provides a spot of increased brilliance on the fluorescent screen. The mesh incorporated in this known type of CRTs, however, causes a decrease in electron gun efficiency, a defocusing of the beam spot on the screen, and halation due to secondary emission from the mesh. Recent efforts in the electronics industry have therefore been directed toward the development of meshless CRTs.
U.S. Pat. No. 4,142,128 to Odenthal reflects an example of such conventional efforts. This patent proposes a box shaped, four element electronic lens for use in both monoaccelerator and postacceleration CRTs. The electronic lens, commonly referred to as a scan expansion lens or deflection amplification lens, defeats many of the limitations of the more conventional meshes. For truly satisfactory display characteristics, however, the lens must measure 10.6 by 6.3 by 2.5 centimeters to provide an eight by 10 centimeter display. This size is far greater than that of the dome mesh, making Odental's lens unusable with the glass envelope of the standard CRT size. Another drawback of the known lens appears in its application to postacceleration CRTs. The exit end electrode of the lens must be electrically connected to the CRT screen in this application, with the consequent difficulties in giving the required voltage withstanding abilities to the lens electrodes.
These drawbacks are absent from the three element lens system described and claimed in U.S. Pat. No. 4,302,704 filed by the instant applicant. Intended for use in postacceleration CRTs, the lens system has three tubuIar or box-like electrodes disposed in axial alignment and electrically insulated from one another. The target side electrode has an end plate which closes its beam exit end and which has an elongate aperture formed therein. The lens system gives the beam a divergent action in one of the orthogonal directions of beam deflection and a doubly convergent action in the other, making it possible to provide a spot that suffers little or no defocusing in the vertical direction.
However, the applicant's prior lens system has proved to have certain inconveniences. One of these is that the intermediate electrode of the lens system has its two pairs of sides convexed and concaved toward both gun and target, with the two outer electrodes being shaped correspondingly to provide insulating gaps of constant width (approximately one millimeter) therebetween. This configuration requires the individual electrodes to be manufactured to very stringent dimensional tolerances for the provision of a lens system of desired performance characteristics. Another is the need for the provision of a shielding electrode to prevent the intrusion of the postaccelerating field into the lens system through the insulating gaps.